.TH STRFTIME 3
.SH NAME
strftime \- generate formatted time information
.SH SYNOPSIS
.ft B
.nf
#include <sys/types.h>
#include <time.h>
.sp
size_t strftime(char *s, size_t maxsize, const char *format,
	const struct tm *timeptr);
.SH DESCRIPTION
The following description is transcribed verbatim from the December 7, 1988
draft standard for ANSI C.
This draft is essentially identical in technical content
to the final version of the standard.
.LP
The
.B strftime
function places characters into the array pointed to by
.B s
as controlled by the string pointed to by
.BR format .
The format shall be a multibyte character sequence, beginning and ending in
its initial shift state.
The
.B format
string consists of zero or more conversion specifiers and ordinary
multibyte characters.  A conversion specifier consists of a
.B %
character followed by a character that determines the behavior of the
conversion specifier.
All ordinary multibyte characters (including the terminating null
character) are copied unchanged into the array.
If copying takes place between objects that overlap the behavior is undefined.
No more than
.B maxsize
characters are placed into the array.
Each conversion specifier is replaced by appropriate characters as described
in the following list.
The appropriate characters are determined by the
.B LC_TIME
category of the current locale and by the values contained in the
structure pointed to by
.BR timeptr .
.TP
.B %a
is replaced by the locale's abbreviated weekday name.
.TP
.B %A
is replaced by the locale's full weekday name.
.TP
.B %b
is replaced by the locale's abbreviated month name.
.TP
.B %B
is replaced by the locale's full month name.
.TP
.B %c
is replaced by the locale's appropriate date and time representation.
.TP
.B %d
is replaced by the day of the month as a decimal number
.RB ( 01 - 31 ).
.TP
.B %H
is replaced by the hour (24-hour clock) as a decimal number
.RB ( 00 - 23 ).
.TP
.B %I
is replaced by the hour (12-hour clock) as a decimal number
.RB ( 01 - 12 ).
.TP
.B %j
is replaced by the day of the year as a decimal number
.RB ( 001 - 366 ).
.TP
.B %m
is replaced by the month as a decimal number
.RB ( 01 - 12 ).
.TP
.B %M
is replaced by the minute as a decimal number
.RB ( 00 - 59 ).
.TP
.B %p
is replaced by the locale's equivalent of the AM/PM designations associated
with a 12-hour clock.
.TP
.B %S
is replaced by the second as a decimal number
.RB ( 00 - 60 ).
.TP
.B %U
is replaced by the week number of the year (the first Sunday as the first
day of week 1) as a decimal number
.RB ( 00 - 53 ).
.TP
.B %w
is replaced by the weekday as a decimal number
.RB [ "0 " (Sunday)- 6 ].
.TP
.B %W
is replaced by the week number of the year (the first Monday as the first
day of week 1) as a decimal number
.RB ( 00 - 53 ).
.TP
.B %x
is replaced by the locale's appropriate date representation.
.TP
.B %X
is replaced by the locale's appropriate time representation.
.TP
.B %y
is replaced by the year without century as a decimal number
.RB ( 00 - 99 ).
.TP
.B %Y
is replaced by the year with century as a decimal number.
.TP
.B %Z
is replaced by the time zone name or abbreviation, or by no characters if
no time zone is determinable.
.TP
.B %%
is replaced by
.BR % .
.LP
If a conversion specifier is not one of the above, the behavior is
undefined.
.SH RETURNS
If the total number of resulting characters including the terminating null
character is not more than
.BR maxsize ,
the
.B strftime
function returns the number of characters placed into the array pointed to
by
.B s
not including the terminating null character.
Otherwise, zero is returned and the contents of the array are indeterminate.
.SH NON-ANSI EXTENSIONS
If
.B SYSV_EXT
is defined when the routine is compiled, then the following additional
conversions will be available.
These are borrowed from the System V
.IR cftime (3)
and
.IR ascftime (3)
routines.
.TP
.B %D
is equivalent to specifying
.BR %m/%d/%y .
.TP
.B %e
is replaced by the day of the month,
padded with a blank if it is only one digit.
.TP
.B %h
is equivalent to
.BR %b ,
above.
.TP
.B %n
is replaced with a newline character (\s-1ASCII LF\s+1).
.TP
.B %r
is equivalent to specifying
.BR "%I:%M:%S %p" .
.TP
.B %R
is equivalent to specifying
.BR %H:%M .
.TP
.B %T
is equivalent to specifying
.BR %H:%M:%S .
.TP
.B %t
is replaced with a \s-1TAB\s+1 character.
.PP
If
.B SUNOS_EXT
is defined when the routine is compiled, then the following additional
conversions will be available.
These are borrowed from the SunOS version of
.IR strftime .
.TP
.B %k
is replaced by the hour (24-hour clock) as a decimal number
.RB ( 0 - 23 ).
Single digit numbers are padded with a blank.
.TP
.B %l
is replaced by the hour (12-hour clock) as a decimal number
.RB ( 1 - 12 ).
Single digit numbers are padded with a blank.
.PP
If
.B HPUX_EXT
is defined when the routine is compiled, then the following additional
conversions will be available.
These are borrowed from the HP-UX version of
.IR date .
.TP
.B %E
The combined ``Emporer/Era'' name and year
(same as
.B %Y ).
This is only available if
.B POSIX2_DATE
is
.I not
defined.
.TP
.B %F
Same as
.BR %B .
(Marked as obsolescent in the HP-UX documentation.)
.TP
.B %N
The ``Emporer/Era'' name.
Typically, this is equivalent to the century
(same as
.B %C ).
.TP
.B %o
The ``Emporer/Era'' year.
Typically, this is equivalent to the year
(same as
.B %y ).
.TP
.B %z
The timezone name or abbreviation.
This is only available if
.B MAILHEADER_EXT
is
.I not
defined.
.SH POSIX 1003.2 EXTENSIONS
If
.B POSIX2_DATE
is defined, then all of the conversions available with
.B SYSV_EXT
and
.B SUNOS_EXT
are available, as well as the
following additional conversions:
.TP
.B %C
The century, as a number between 00 and 99.
.TP
.B %u
is replaced by the weekday as a decimal number
.RB [ "1 " (Monday)- 7 ].
.TP
.B %V
is replaced by the week number of the year (the first Monday as the first
day of week 1) as a decimal number
.RB ( 01 - 53 ).
The method for determining the week number is as specified by ISO 8601
(to wit: if the week containing January 1 has four or more days in the
new year, then it is week 1, otherwise it is the highest numbered
week of the previous year (52 or 53)
and the next week is week 1).
.LP
The text of the POSIX standard for the
.I date
utility describes
.B %U
and
.B %W
this way:
.TP
.B %U
is replaced by the week number of the year (the first Sunday as the first
day of week 1) as a decimal number
.RB ( 00 - 53 ).
All days in a new year preceding the first Sunday are considered to be
in week 0.
.TP
.B %W
is replaced by the week number of the year (the first Monday as the first
day of week 1) as a decimal number
.RB ( 00 - 53 ).
All days in a new year preceding the first Monday are considered to be
in week 0.
.LP
In addition, the alternate representations
.BR %Ec ,
.BR %EC ,
.BR %Ex ,
.BR %Ey ,
.BR %EY ,
.BR %Od ,
.BR %Oe ,
.BR %OH ,
.BR %OI ,
.BR %Om ,
.BR %OM ,
.BR %OS ,
.BR %Ou ,
.BR %OU ,
.BR %OV ,
.BR %Ow ,
.BR %OW ,
and
.B %Oy
are recognized, but their normal representations are used.
.SH VMS EXTENSIONS
If
.B VMS_EXT
is defined, then the following additional conversion is available:
.TP
.B %v
The date in VMS format (e.g. 20-JUN-1991).
.SH MAIL HEADER EXTENSIONS
If
.B MAILHEADER_EXT
is defined, then the following additional conversion is available:
.TP
.B %z
The timezone offset in a +HHMM format (e.g. the format necessary to
produce RFC-822/RFC-1036 date headers).
.SH ISO DATE FORMAT EXTENSIONS
If
.B ISO_DATE_EXT
is defined, then all of the conversions available with
.BR POSIX2_DATE,
.BR SYSV_EXT,
and
.B SUNOS_EXT
are available, as well as the
following additional conversions:
.TP
.B %G
is replaced by the year with century of the ISO week number (see
.BR %V ,
above) as a decimal number.
.TP
.B %g
is replaced by the year without century of the ISO week number,
as a decimal number
.RB ( 00 - 99 ).
.PP
For example, January 1, 1993, is in week 53 of 1992. Thus, the year
of its ISO week number is 1992, even though its year is 1993.
Similarly, December 31, 1973, is in week 1 of 1974. Thus, the year
of its ISO week number is 1974, even though its year is 1973.
.SH SEE ALSO
.IR time (2),
.IR ctime (3),
.IR localtime (3),
.IR tzset (3)
.SH BUGS
This version does not handle multibyte characters or pay attention to the
setting of the
.B LC_TIME
environment variable.
.LP
The ``appropriate'' values used for
.BR %c ,
.BR %x ,
are
.B %X
are those specified by the 1003.2 standard for the POSIX locale.
.SH CAVEATS
The pre-processor symbol
.B POSIX_SEMANTICS
is automatically defined, which forces the code to call
.IR tzset (3)
whenever the
.B TZ
environment variable has changed.
If this routine will be used in an application that will not be changing
.BR TZ ,
then there may be some performance improvements by not defining
.BR POSIX_SEMANTICS .
.SH AUTHOR
.nf
Arnold Robbins
.sp
INTERNET: arnold@skeeve.com
.fi
.SH ACKNOWLEDGEMENTS
Thanks to Geoff Clare <gwc@root.co.uk> for helping debug earlier
versions of this routine, and for advice about POSIX semantics.
Additional thanks to Arthur David Olsen <ado@elsie.nci.nih.gov>
for some code improvements.
Thanks also to Tor Lillqvist <tml@tik.vtt.fi>
for code fixes to the ISO 8601 code.
Thanks to Hume Smith for pointing out a problem with the ISO 8601 code
and to Arthur David Olsen for further discussions.
